The present invention relates to an earth-acid metal (mainly, niobium or tantalum) powder from which a capacitor having a large capacitance per unit mass and good leakage current characteristics can be produced. More specifically, the present invention relates to a niobium powder, a tantalum powder and a niobium-tantalum alloy powder, each containing a specific amount of zirconium, and also relates to a sintered body using the powder and a capacitor using the sintered body.
Capacitors for use in electronic instruments such as potable telephone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance. In the tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of these tantalum capacitors, it is necessary to increase the mass of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the specific surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering and therefore, impregnation of the cathode agent in the later process becomes difficult. In order to solve these problems, a method of decreasing closed pores at the stage of sintering and a method of manufacturing a capacitor using a material having a dielectric constant larger than that of tantalum are being studied. The material having a larger dielectric constant includes niobium.
JP-A-55-157226 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a method for producing a sintered element for capacitors, where an agglomerated powder of niobium fine powder particles (primary powder) having a particle size of 2.0 xcexcm or less is molded under pressure, a sintered body of the molded article is cut into fine pieces, a lead part is joined therewith and then these are again sintered. However, this patent publication does not disclose in detail the properties of the capacitor manufactured using the sintered element.
U.S. Pat. No. 4,084,965 discloses a capacitor using a niobium powder of 5.1 xcexcm obtained by hydrogenating and pulverizing a niobium ingot. However, the niobium sintered body has a large leakage current value (hereinafter simply referred to as xe2x80x9cLC valuexe2x80x9d) and the practicability thereof is low.
JP-A-10-242004 (U.S. Pat. No. 6,115,235) discloses a technique of improving the LC value, for example, by partially nitriding a niobium powder. However, in the case of manufacturing a high-capacitance capacitor from a niobium sintered body using a niobium powder having a fine particle size, a capacitor having a peculiarly large LC value is produced in some cases.
Accordingly, the object of the present invention is to provide a powder for capacitors, which can provide a capacitor having a large capacitance per unit mass and a small leakage current value. The object of the present invention includes providing a sintered body using the powder and a capacitor using the sintered body.
As a result of extensive investigations to solve the above-described problems, the present inventors have found that by incorporating a specific amount of zirconium into niobium, tantalum or a niobium-tantalum alloy, a large specific surface area can be maintained even if the powder used for sintering has a fine average particle size and that when a capacitor is manufactured using this sintered body, a high-capacitance and stable capacitor having a low LC value can be obtained. The present invention has been accomplished based on these findings.
More specifically, the present invention relates to the following powder for capacitors, a sintered body thereof, and a capacitor using the sintered body.
1. A powder for capacitors, which contains zirconium and mainly comprises niobium and/or tantalum.
2. The powder for capacitors as described in 1 above, which contains from 0.01 to 15 atom % of zirconium and mainly comprises niobium and/or tantalum.
3. The powder for capacitors as described in 1 or 2 above, which mainly comprises niobium.
4. The powder for capacitors as described in 1 or 2 above, which mainly comprises tantalum.
5. The powder for capacitors as described in 1 above, which mainly comprises a niobium-tantalum alloy.
6. The powder for capacitors as described in any one of 1 to 5 above, wherein the average particle size is from 0.2 to 5 xcexcm.
7. The powder for capacitors as described in any one of 1 to 5 above, wherein the specific surface area is from 0.5 to 15 m2/g.
8. The powder for capacitors as described in any one of 1 to 5 above, wherein a part of niobium and/or tantalum is combined with at least one element selected from nitrogen, carbon, boron or sulfur.
9. The powder for capacitors as described in 8 above, wherein the amount of the combined element above is 50xcx9c200,000 ppm.
10. A powder for capacitors, having an average particle size of from 20 to 500 xcexcm, which is obtained by granulating the powder for capacitors described in any one of 1 to 9 above.
11. A sintered body using the powder for capacitors described in any one of 1 to 10 above.
12. The sintered body as described in 11 above, wherein the specific surface area is from 0.5 to 5 m2/g.
13. A capacitor fabricated from the sintered body described in 11 or 12 above as one part electrode, a dielectric material formed on the surface of the sintered body, and another part electrode provided on the dielectric material.
14. The capacitor as described in 13 above, wherein the dielectric material contains a niobium oxide and/or a tantalum oxide.
15. The capacitor as described in 14 above, wherein the niobium oxide and/or tantalum oxide is formed by electrolytic oxidation.
16. The capacitor as described in 13 above, wherein the another part electrode is at least one material selected from an electrolytic solution, an organic semiconductor or an inorganic semiconductor.
17. The capacitor as described in 16 above, wherein the another part electrode is formed of an organic semiconductor and the organic semiconductor is at least one organic semiconductor selected from the group consisting of an organic semiconductor comprising a benzopyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane, and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant into a polymer comprising two or more repeating units represented by the following formula (1) or (2): 
(wherein R1 to R4, which may be the same or different, each represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R5 is present only when X is a nitrogen atom and represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms, and each of the pairs R1 and R2, and R3 and R4 may combine with each other to form a ring).
18. The capacitor as described in 17 above, wherein the organic semiconductor is at least one member selected from polypyrrole, polythiophene and substitution derivatives thereof.